1. Field of the Invention
The present invention relates to a quenching method, more precisely, to a quenching method improved in point of reducing quenching distortion.
2. Description of the Related Art
In the field of automobile industry, quenching distortion of gears results in a gear-to-gear meshing error, therefore causing noises and damage to gear teeth. In the field of bearings, their quenching distortion results in increasing the grinding allowance in the subsequent process, therefore causing the productivity reduction.
Heretofore, an oily agent well workable even at high oil temperatures, so-called hot oil (having a kinematic viscosity at 100° C. of from 10 to 30 mm2/sec or so) is used for reducing quenching distortion. Hot oil of the type is highly effective for reducing quenching distortion, but is often problematic in that the articles quenched therein could not be cooled rapidly and their hardness is not good. As a result, the teeth of the quenched articles may be damaged or broken and the fatigue life thereof is not long.
On the other hand, when cold oil (having a kinematic viscosity at 100° C. of at most 6 mm2/sec or so) is used, the articles quenched therein may have a high hardness but are problematic in that their distortion increases.
As in the above, the hardness of quenched articles and the quenching distortion thereof are in the trade-off, and cold oil is used when the hardness of quenched articles is considered important but hot oil is used when the reduction in quenching distortion is considered important.
Recently, however, for obtaining quenched articles that are distorted little and have a desired hardness, some methods have been proposed in which the quenching bath used is kept pressurized in a certain period of time during the cooling step. For example, JP-A-61-79716 discloses a method in which the quenching bath used is kept under atmospheric pressure until the quenched articles reach around the Ms point thereof and then it is pressurized at around the Ms point to thereby control the boiling point of the quenching oil therein. This is for retarding the cooling rate in the bath. JP-A-4-28818 discloses a method in which the quenching bath used is kept under reduced pressure up to around the Ms point thereof and then pressurized at around the Ms point to thereby control the boiling point of the quenching oil therein. This is also for retarding the cooling rate in the bath. JP-A-8-60234 discloses a method in which the quenching bath used is kept under pressure until it reaches the characteristic temperature thereof, and, after it has become lower than the characteristic temperature, its pressure is gradually lowered to atmospheric pressure or to around atmospheric pressure. In any of these methods, the quenched articles could have the desired hardness, but there is still room for improvement in these methods in point of reducing the quenching distortion therein.